William is 4 times as old as Gabriela and is also 15 years older than Gabriela. How old is Gabriela?
We can use the given information to write down two equations that describe the ages of William and Gabriela. Let William's current age be $w$ and Gabriela's current age be $g$ $w = 4g$ $w = g + 15$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $g$ , and both of our equations have $w$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4g$ $-$ $ (g + 15)$ which combines the information about $g$ from both of our original equations. Solving for $g$ , we get: $3 g = 15$ $g = 5$.